Bottles for compressed gases of AU6MGT

ABSTRACT

Bottles for compressed gases of AU6MGT with good mechanical properties and resilience, coupled with resistance to intercrystalline corrosion and to corrosion under tension. These bottles are obtained by drawing on a mandrel, in the absence of supports or, better still, by the combined drawing of an extruded blank, quenched and aged, comprising a cylindrical part and a base, followed by tapering of the end at a temperature of from 350° to 400° C, the propagation of heat towards the cylindrical part being prevented by means of a cooling ring, and finally by quenching in cold water until cooling is complete. The bottles obtained by this method are extremely safe to use and satisfy the most stringent requirements, in particular in the field of aviation, space vehicles and in every case where it is desired to combine minimal weight with good resilience and high resistance to corrosion.

This invention relates to containers of aluminum-based alloy which are able to withstand high internal pressures, and more particularly to bottles for compressed gases.

It is known that bottles for compressed gases can be produced from an aluminum-based alloy known by the symbol AU6MGT according to French Standards AFNOR A 02001 and A 02002. This alloy has the following composition:

    ______________________________________                                         Copper            5 to 7%                                                      Manganese         0.05 to 0.50%                                                Magnesium         0.10 to 0.50%                                                Titanium          0.05 to 0.25%                                                Iron              0.50%                                                        Silicon           0.30%                                                        Aluminum          balance to 100%                                              ______________________________________                                    

French Pat. No. 1,379,764 for example describes the manufacture of bottles for compressed gases from AU6MGT.

The bottles obtained by the process described in this patent do not have the necessary resilience properties to satisfy the extremely stringent fragility tests applied under certain regulations.

The tests in question include, in particular, bursting tests under internal air pressure without shattering of the bottle, and crushing or flattening tests between two standard wedges placed perpendicularly of the generatrices of the bottle.

The use of AU6MGT treated as described in the aforementioned patent, i.e. solution heat treated for 1 to 3 hours at 510°/540°C, quenched in water and tempered at 175° C over a period of 24 hours, always leads to negative results for these two tests.

None of the other aluminum alloys which have been tested for manufacturing bottles for compressed gases satisfies the following four requirements at the same time:

1. A high resilience, characterized for example by the two above tests or by modern fragility tests, such as studying crack proopagation energy.

This test, known as MANLABS or PHYSMET, from the name of the American Company which manufactures the necessary equipment, has been the subject of considerable development over the past few years in aeronautical laboratories and in the advanced research laboratories of Aluminum Pechiney. It uses test specimens of the CHARPY V type, in which the conventional notch is extended by a fatigue crack, and comprises determining the breaking energy by impact flexing or by gradual flexing. By varying the depth of the cracks, it is possible to trace the curve giving the value of the resiliences as a function of the depth of the crack.

By extrapolating this curve, it is possible to deduce the energy absorbed by a bar containing an infinitesimally small fatigue crack.

This energy, expressed as E_(co), is considered to be indicative of the crack propagation resistance of materials. It is more or less lower than the energy E_(nc) required to break a test specimen of CHARPY V type notched by machining, but not cracked.

2. A very high resistance to intercrystalline corrosion.

3. A high resistance to corrosion under tension.

4. Good mechanical properties allowing a bottle of minimum weight to be obtained.

We have found that AU6MGT can be subjected to combined heat and mechanical treatment in such a way that it is possible to obtain bottles showing all four of the aforementioned properties at the same time.

The present invention relates to containers which have to withstand high internal pressures and more particularly, to bottles for compressed gases of AU6MGT, distinguished by an ultimate tensile strength of more than 46 hbars, a yield strength of more than 39 hbars, an elongation of more than 15% (with a distance between marks of 5.65 So), So being the initial section of the test specimen, a crack propagation energy E_(co) of more than 6 joules (as measured by the resilience test on test specimens cracked by fatigue, using 10 × 5 mm test specimens), an excellent resistance to intercrystalline corrosion and a remarkable resistance to corrosion under tension under a stress equal to or greater than 85% of the yield strength.

The invention also relates to a method of manufacturing containers which have to withstand high internal pressure, more especially bottles for compressed gases of AU6MGT, showing the properties which have just been described from an extruded blank comprising the cylindrical part and the base.

According to the invention, the blank is subjected successively to a solution heat treatment for 1 to 4 hours at 510°/540° C, to quenching in water, to natural aging for several days (betwen 3 and 12 days, but preferably for 8 days) at ambient temperature which can be accelerated by heating for a few hours at a temperature below 130° C, preferably for 24 hours at a temperature of 100° C, and then to cold-working at a rate of from 3 to 50% by drawing the blank first unsupported and then on a mandrel which provides the blank with its final dimensions by reducing its initial diameter by from 2 to 10%.

The invention also relates to a method of tapering under heat by heating to 350°/400° C that section intended to form the taper with the interposition of a water-circulation ring preventing the heat from spreading towards the remaining cylindrical part, followed by quenching in cold water of the tapered bottle until it has completely cooled.

As a result of efforts to improve bottles of a AU6MGT manufactured by known methods, I have found that the cold-working of a tapered bottle by drawing in the absence of any support produced extremely satisfactory mechanical characteristics, but gave rise to the formation of creases and bulges near the base, altering the appearance of the end product. By contrast, drawing of the blank on a mandrel before tapering produced bottles without any external faults, but with less elongation than that produced by drawing in the absence of any support (approximately 12%), the crack propagation energy E_(co) remaining extremely high.

I have now perfected a process which obviates the disadvantages of these two methods while retaining their respective advantages, namely a process in which drawing in the absence of a support is combined with drawing on a mandrel and which enables remarkable mechanical properties to be obtained without any adverse effect upon resistance to corrosion.

The invention is described in more detail in the following with reference to the accompanying drawing, wherein

FIG. 1 diagrammatically illustrates the combined drawing system;

FIG. 2 shows the initial phase; and

FIG. 3 shows the final phase of the tapering process.

FIG. 1 diagrammatically illustrates the combined drawing system, in which an extruded blank composed of AU6MGT, quenched and aged in the same way as described above, is forced through a drawing plate 4 by means of a ram 3 cooperating with a base plate 2.

The initial and final external diameters 11 and 12 of the cylindrical part are such that deformation takes place for the most part in the absence of a support and, to a far lesser extent, during passage through the drawing plate 4, so that the elongation of the tube is just sufficient to eliminate the external aesthetic faults produced by drawing in the absence of a support and to give the optimum coldworking level.

The optimum characteristics of the bottles are obtained for a difference of 4 to 10 mn between the diameter of the ram and the initial bore of the extruded blank for a decrease in diameter of from 2 to 10% in the initial diameter D1 (and preferably between 4% and 6%), for a cold-working level, expressed by the ratio S₁ - S₂ /, S₁ of from 5 to 30% (preferably from 15 to 20%), S₁ being the initial section of the blank and S₂ the final section after drawing.

FIG. 2 diagrammatically illustrates the initial phase of the tapering process. The end 5 of the cylindrical part 7 is heated by any known means, such as a bath of molten lead, induced current or any other method providing for rapid, local heating to a temperature in the range of from 350° to 400° C.

A cooling ring or collar 6, which can be in the form of a quadrangular copper tube with a cooling fluid flowing through it (such as water, brine, oil or compressed air), arranged tightly around the blank 1, while maintaining the possibility of sliding, enables heating of the cylindrical part situated immediately behind to be avoided. At the same time the blank 1 is forced into the heating mold 7, while the cooling system 6 slides progressively upwards up to the limit of the taper zone 8.

FIG. 3 illustrates the final phase of the tapering operation. The reduction in the diameter of the tapered part 8 relative to the cylindrical part 5 from which it was produced, is reflected in a corresponding increase in wall thickness, which compensates very largely for any reduction in the characteristics of the tapered part as a result of its heating to 350° to 400° C.

The tapering operation takes at least two minutes and, as soon as it is complete, the bottle as a whole is quenched in cold water until it has completely cooled. Tests carried out both on the taper and on the adjacent tubular part have not revealed the slightest susceptibility to intercrystalline corrosion.

The finishing operations, in particular the fitting of the valve, are carried out in the usual way.

The following characteristics for example were obtained with a batch of bottles of AU6MGT with the chemical composition defined on page 1 and manufactured by the following process:

solution heat treatment at 530° + 2° C for 2.5 hours (in a salt bath)

quenching in cold water ⁻ ¹ (15° C)

aging for 8 days at 18° to 22° C

20% combined drawing with a reduction in diameter of 4.5%

tapering at 380° C in a lead bath (heating time 30 seconds and total tapering time 1 minute 30 seconds).

    ______________________________________                                         ultimate tensile strength                                                                       minimum = 47 hours                                            yield strength   minimum = 40 hbars                                            elongation       minimum = 15.5% (with 5.65 So)                                E.sub.co         6.10 joules                                                   ______________________________________                                    

no sign of intercrystalline corrosion after testing in the reagent NACl + H₂ O₂ (in accordance with Standard AIR 9050/C).

no break after 3 months' testing of 5 test specimens for corrosion under tension, using the following procedure:

standardized aeronautical bath A₃ (according to Standard AIR 0754/A), consisiting of 3 kg of sodium chloride, 125 g of boric acid and 19 g of disodium phosphate in 100 liters of distilled water, the pH being adjusted to between 8 and 8.2 by the addition of a saturated solution of sodium carbonate in distilled water,

alternate immersion and emersion,

flexing stress equal to 86% of the yield strength.

It is possible by virtue of the invention to obtain bottles for compressed gases of AU6MGT which are able to withstand the most stringent resilience and corrosion tests.

It is also possible by virtue of the invention to obtain bodies of filters or bodies of jacks and, generally, tubular bodies of AU6MGT, optionally tapered, subjected to high internal pressure while, at the same time, being light in weight and safe to use. 

I claim:
 1. In a method of manufacturing bottle capable of withstanding a high internal pressure, in which said bottle is formed of an aluminum based alloy consisting essentially of, in per cent by weight, approximately, 5 to 7% copper, 0.05 to 0.50% manganese, 0.10 to 0.50% magnesium, 0.05 to 0.25% titanium, 0.50% iron, 0.30% silicon, the balance to 100% of aluminum and extruded to form a blank having a cylindrical part and a base at one end of the cylindrical part, the improvement comprising, the steps of subjecting the blank successively to a solution heat treatment for 1 to 4 hours at 510°/540° C, to quenching in water, to natural aging for several days at ambient termperature and then to cold working to a level of from 5 to 30%, by drawing in the absence of a support and drawing on a mandrel, which brings the blank to its final dimensions by reducing its initial diameter by 2 to 10%.
 2. The method as claimed in claim 1 in which a taper is formed in the end of the blank opposite the base by heating to 350°/400° C that part of the blank which is intended to form the taper with interposition of a water-circulation ring preventing the heat from spreading towards the remaining cylindrical part, followed by quenching the tapered bottle in cold water until it has completely cooled.
 3. Bottles produced by the method of claim 2 characterized by an ultimate tensile strength of more than 46 hbars, a yield strength of more than 39 hbars, an elongation of more than 15% with a distance between marks of 5.65 So, So being the initial section of the test specimen, a crack propagation energy E_(co) of more than 6 joules, as determined by the PHYSMET test on test specimens measuring 10 × 5 mn, an excellent resistance to corrosion under tension under a stress equal to or greater than 85% of the yield strength.
 4. The method as claimed in claim 1 wherein aging of the extruded blank is accelerated by heating for a few hours at a temperature below 130° C.
 5. The method as claimed in claim 1 in which the blank is subjected to natural aging for 3 to 12 days.
 6. The method as claimed in claim 1 in which the blank is cold worked to a level of from 15 to 20%.
 7. The method as claimed in claim 1 in which the initial diameter of the blank is reduced by 4 to 5%.
 8. The method as claimed in claim 4 in which the aging is accelerated by heating the blank for about 24 hours at a temperature of about 100° C. 